Separation of esters



April 10, 1956 D. l.. coTTLE SEPARATION oF EsTERs Filed Aug. 18, 1951 Nom? van lv ^1oU YUO Q lv omNI IOu Ll M Ion( Y m. cd1 r mtwm mbw No .YOWNI N NQ wl nog. U mllhw m..\ w amhnw ANU umlllimm .(3 wfzle HON. (Ov (i uhmm .mQoN Qormwhmw l vow IOU m Dehner Llottle, nventor Clttorne United States Paten-t sEPAnATroNor EsrERs Dehner L. Cottle, Highland Park,.N. I., assigner to Esso Research and Engineering Company, a corporation of Delaware Application August i8, 1951, SeriaINo'. 242,486

2 Claims. (Cl. 260-499) This invention relates to a novel method of separating esters in a mixture prepared by the esteriiication of` olens with monocarboxylic acids in the presence of a sulfurie acid catalyst.

The esteriiication of olens With monocarboxylic acids utilizing sulfuric acid as a catalyst is well known in the art (see, for example, U. S. Patent No. 2,006,734). Olenic material, preferably in the liquid phase, is introduced into a reaction unit Where it is brought into'intimatel contact with an organic acid and catalyst by agitation, for example, or by other dispersing means.

The isolation of esters formed from alcohols and organic acids is a simple process, largely because the amount of sulfuric acid used as catalyst is too small a cencentration to materially damage the product during its separation.

The same is not true of processes in which olelns are directly esterified with organic acids.

Whereas the alcohol process for esters may use about one wt. per cent sulfuric acid catalyst, a direct olefin esterication process uses up to Wt. per cent sulfuric based on acetic acid alone and a minimum of aboutl 5%. Thus the i'mal reaction mixture to be separated comprises principally ester, sulfuric acid, and carboxylic acid. The separation of the esters is complicated by the fact that the concentration of sulfuric acid is high enough to catalyze decomposition of the esters, a problem not encountered in alcohol esteriiication processes. That this problem is recognized can be seen from the article in Industrial Engineering Chemistry, January 1938, page 57,

which states: The nal step of the process isf the isola-V c-f the ester in a salable form and the unreacted Direct distillation of the mation acid in condition for reuse.

terial is generally unsatisfactory because the small amountV of sulfuric acid present concentrates along with the ester as the hydrocarbon is removed and promotes decomposition. The conditions mentioned in this article are those obtained after the bulk of the sulfuric acid has been separated in the lower layer of a two-phase system. Thus the problem is even more marked in a homogeneous product system comprising principally ester, carboxylic acid and sulfuric acid.

Another problem in the separation of theA product arises from the fact that the esters often boil close to or above the boiling. points of the carboxylic acids in the mixture.-

This tends to make the ester separation diflicult or expensive, or both.- Fractional distillation of. the reaction mixture does` not satisfactorily separate sec-butyl acetate B. P. 112 C. from acetic acid B. P. 118 C. because of the closeness of their boiling. points. Addition of sucient water to the reaction mixture to form a 2-1ayer system causes some separation' of acetic acid from sec-butylY acetate, but the acetic acid and sulfuric acid catalyst are thereby diluted with Water and must be reconcentrated before being recycled to the reaction unit.

This invention provides a method for overcoming thebet'orementioned difculties. The method comprises, among other things, adding a small amountofwater and 21,741,532 Patented Apr. 10, 1956 ester, C. otorzie, water Ethyl acetate 77. 1 70. 4 6. 1 Isopropyl acetate 91. 0 77. d 6. 2 Ethyl proplouat 99. 15 81. 2 10' sec-Butyl acetate- 112 86.6 19:4 Ethyl butyrat.e 121. 6 87. 9 21. 5 see-Amyl acetate 133 92 32. 2

While excess Water can be azeotroped out by recycleA of ester, this is costly, and it is definitely preferable not to" utilize excess water.

The excess carboxylic acid can be added before the reaction with the olefin or after reaction, but in any casel should oe present during the distillation of the ester. The quantity utilizedY is approximately that sufcient to replace the carboxylic acid used up in the esterication process. For example, where the mol ratio of acetic acid to olefinV is 1/ 1 and the yield of ester is 54 mol per cent, an addition of 0.54'mol of acetic a'cidper mol of acetic' acid originally present would be made. If l0 wt. per cent sul'- furic acid'l based on acetic acid was used as a catalyst, the' sulfuric concentration at the end of the reaction would be" 20'wt. per cent, sincev approximately half of the acetic acid was converted to ester. Addition of 0.54 mol acetic acidt would restore' the wt. per cent' value of the sulfuric acid' to` l0 wt. per cent, and thus' make it a more satisfactory solution from which to distill the ester.

A mixture or carboxylic acid and Water could be uti-A lized with a concentration of each so chosen as to conf form to the preceding requirements.

By the term relatively anhydrous sulfuric acid it is meant sulfuric acid having a minimum concentration of about Wt. per cent on an organic-free basis so' as to' be usable' as a catalyst in thev esteric'ation reaction, i. e., does' not contain excessive. water which would destroy its' catalytic elciency.

The two added components, i. e., water and carboxylic acid, cooperate in a surprising manner to facilitate ester separation. The addition of water by favoring ester azeotrope formation reduces the boiling point of ester.Y to make it more easily separable. it would be expected,` however, that the addition of water to a system containing as much as 10. wt. percent concentrated sulfuricr acid Would result in severe processdiflicults and a wet sulfuric acid not suitableifor use in esteriiication. The minimum concentration of sulfuric acid itself that can be utilized asa catalyst is about' 65v Wt', percent. Actually, however, the' carboxylic acid added, in addition to preventing ester decomposition, aids in the displacement of the water from the sulfuric acid to the ester aze'ot'rope at temperatures' utilized, preferably below 100 C. The carboxylic acid also contributes to the preventing of excess'ive temperature ris'e: during theV distillation. The carboxylic acid-sulfuric acid mixture is left as a residue from the distillation in a form suitable for recycling to esterication. sulfuric acid of 96% concentration is obtained even in the presence of this added water, a'

3 truly surprising result. The significance of this is illustrated by the fact that a decrease in eective sulfuric acid concentration from 80 to60% (on an organic-free basis) decreases the yield of secondary butyl acetate from n-butylenes and acetic Vacid from 56 to 11 mol percent.

The oletins which can be used in the esterifcation reaction include C2 to Cs monooleiins, i. e., ethylene, propylene, l-butene, 2-butene, l-pentene, isopentene, 2- pentene, 1-hexene, isohexene, 2-hexene, as well as isoprene, butadiene,'and styrene or mixtures thereof. YThere is no exact limit to the molecular weight or number of carbon atoms in the olefin used.

The acids which can be used for esteriiication oy the olelins include a Vwide variety of C1 to Cs aliphatic monobasic acids. Acids which may be used include formic acid, aceticacid, propionic acid, butyric acid, isobutyric acid. valerie acid, caproic acid, capric acid.

As a general rule, it is best to use this improved Vester separation process for esters having from 3 to l2 carbon atoms and which Vform Water azeotropes which distill below the temperatures of the acids utilized.

This process is especially adapted for use in the separation of secondary butyl acetate, which is normally separable from the indicated mixtures only with extreme difficulty.

The olefin esteriication reaction mixtures separated comprise principally, as explained above, carboxylic acid, ester, and sulfuric acid, and are thus largely independent of the esteriiication variables themselves. Since the latter are well known and constitute no part of this invention, they need not beelaborated upon here.

This invention will be better understood by reference to the liow diagram shown in the drawing. The Vdescription deals with secondary butyl acetate prepared from normal butylenes and acetic acid utilizing sulfuric acid as the catalyst.

In the system shown a hydrocarbon stream consisting of normal butylenes admixed with butanes and smaller amounts of C3 and Cs hydrocarbons enters esterification zone 1 through line 2. Acetic acid admixed with sulfuric acid of 70l00% strength on an organic-free basis enters esteri'ication zone 1 through line 3. A countercurrent contacting is thereby obtained. The temperatureV is maintained between 70 and 160 C., and a pressure of from 80 to 700 p. s. i. The reaction mixturecomprising principally secondary butyl acetate, acetic acid, sulfuric acid, and unconverted hydrocarbons, is lwithdrawn from the esteriiication zone 1 through line 4 to flash drum 5. The unconverted hydrocarbons, principally Css, are taken overhead through line 6. The residual mixture taken olf from flash drum 5 comprises principally acetic acid, secondary butyl acetate, and sulfuric acid, and is fed from line to distillation tower 3. Acetic acid and water are added to the distillation tower 8 through line 9, although these may be fed in separately.

Suiiiicent water is added to makeup the secondary butyl Y acetate-Water azeotrope, which-has a boiling point of 87 C. and a composition of 19.4 wt. percent water and 80.6 wt. percent ester. The acetic acid is added in an amount sufficient to malte up the acid utilized in the esterication reaction mixture, e. g., 0.45 mol of acetic acid. The secondary butyl acetate-water' azeotrope is taken overhead through line 10 to separation drum 1l, where the azeotrope breaks into two phases, an upper ester phase and a lower water phase. The puried ester product is withdrawn through line i2. VThe Water can be discarded or utilized for volatilizing more ester. The bottoms from the distillation, consisting largely of acetic acid and sulfuric acid of over on an organic-free basis and somev ester, is returned through line 13 into line Sto esteriiication zone 1.

Makeup components can be added as needed. The ester is further worked up, if desired, by conventional processes. Y

4 The following examples provide additional details of the process and highlight its advantages:

Example I A reaction mixture comprising 25 mols of acetic acid, 5 mois of sulfuric acid, and 15 mols of isopropyl acetate had admixed 10 mols'of water and 18 mols of acetic acid. The mixture was distilled in a continuous column at a 10/3 reflux ratio at a feed rate of 600 cc./hour with a still head temperature of 77 C. 63% of the ester was recovered as overhead, with less than 8% decomposed to propylene. Sulfuric acid obtained in the bottoms was sulfuric acid on an organic-free basis. A batch distillation of a similar mixture in which the sulfuric acid was 31% on an organic-free basis gave a 96% sulfuric acid in the residue. Six per cent of the isopropyl acetate was converted to propylene.

These examples illustrate how the sulfuric acid in the presence lof acetic acid and water was dried by the azeotropic distillation which at the same time yielded good separations of the desired'product.

Example Il A mixture of 200 g.kof secondary butyl acetate, 34.2 g. of acetic acid, 6 g; of sulfuric acid, and 12.3 g. of Vwater was distilled through a 20-plate Oldershaw column. 80% of the secondary butyl acetate and 96.5% Vof acetic acid was recovered. The sulfuric acid bottoms Was 96% on an organic-free basis, again pointing out how the dry sulfuric acid was obtained in the presence` of Water and acetic acid` t This process has particular advantages in the production of sec-butyl acetate. separated from acetic acid at some stage by azeotropic distillation with water. There is no other way excepting an expensive chemical treatment which means additional expense in the ultimate recovery of the acetic acid. The discovery that sulfuric acid in the presence of added acetic acid can be recovered dry by an azeotropic distillation makes it possible to separate ester not only from sulfuric acid but also from acetic acid in one operation.

Y The process also has value when any ester so prepared from an oletin has a'boiling point such that the ester may be azeotroped from the reaction mixture satisfactorily with water. For examplean ester distilling at 200 C. could not be removed by distillation from a mixture of sulfuric and acetic acid without first distilling the acetic acid. However, addition of Water would allow the ester to be removed at some temperature below C. leaving the acetic acid and relatively dry sulfuric acid behind. v

it will be understood further that the foregoing examples and apparatus have been given merely for purposes of illustration, but that other modifications of the present invention are possible without departing from the scope of the appended claims.

What is claimed is:

l. in a continuous process for separating an ester from a reaction mixture produced in anV esteriiication step by reaction of a C2 t Cs monooleiin with a C1 to Cs aliphatic monocarboxylic acid in the presence-of about 5 to l() Weight percent of relatively anhydroussulfuric acid based on the monocarboxyiic acid, said ester beingy one capable of forming a water azeo'trope which distills below the temperature'of the monocarboxylic acid used, the improvement which comprises withdrawing the reaction mixture from the esteriiication step, discharging unconverted olefin from the Withdrawn mixture, providing an excess of the free monocarboxylic acid in the re-Y sulting olefin-free mixture, adding to said olefin-free mixture 'a quantity of water about sufhcient to form a water azeotrope with lall the ester formed, heating Vthe resulting aqueous mixture in a distillation zone, withdrawing an ester-water azeotrope as a distillate fraction from said distillation zone, also withdrawing from said distillation Sec-butyl acetate must be,

zone a bottoms fraction containing the free monocarboxylic acid and relatively anhydrous sulfuric acid, recycling said bottoms fraction to the esterication step, and separating the distillate fraction into an aqueous phase and an organic ester phase.

2. In a continuous process for separating secondary butyl acetate from a reaction mixture produced in an esteriiication step by reaction of a normal butylene with acetic acid in the presence of about 5 to 10 weight percent of sulfuric acid having a minimum concentration of about 75 weight percent on an organicfree basis, the improvement which comprises continuously withdrawing the reaction mixture from the esteriication step, continuously ashing unconverted butylene from the withdrawn mixture, continuously adding to the reaction mixture a further quantity of acetic acid in an amount approximately suicient to replace the acetic acid used up in the esterication step, also continuously adding to the withdrawn mixture a quantity of water about sucient to form a water azeotrope with all the ester formed, continuously heating the resulting aqueous mixture in a distillation zone, continuously withdrawing au ester-water azeotrope as a distillate fraction from said distillation zone, also continuously withdrawing from said distillation zone a bottoms fraction containing the acetic acid and sulfuric acid having a minimum concentration of weight percent on an organic-free basis, recycling said bottoms fraction to the esten'cation step, and separating the azeotropic distillate into an aqueous phase and an organic ester phase.

References Cited in the file of this patent UNITED STATES PATENTS 1,898,737 Merley Feb. 21, 1933 1,926,189 Ayres Sept. 12, 1933 2,006,734 Edlund et al. July 2, 1935 2,079,652 Davis et al May 11, 1937 2,438,300 Schuiepp Mar. 23, 1948 

1. IN A CONTINUOUS PROCESS FOR SEPARATING AN ESTER FROM A REACTION MIXTURE PRODUCED IN AN ESTERIFICATION STEP BY REACTION OF A C2 TO C6 MONOOLEFIN WITH A C1 TO C8 ALIPHATIC MONOCARBONBOXYLIC ACID IN THE PRESENCE OF ABOUT 5 TO 10 WEIGHT PERCENT OF RELATIVELY ANHYDROUS SULFURIC ACID BASED ON THE MONOCARBOXYLIC ACID, SAID ESTER BEING ONE CAPABLE OF FORMING A WATER AZEOTROPE WHICH DISTILLS BELOW THE TEMPERATURE OF THE MONOCARBOXYLIC ACID USED. THE IMPROCEMENT WHICH COMPRISES WITHDRAWING THE REACTION MIXTURE FROM THE ESTERIFICATION STEP, DISCHARGING UNCONVERTED OLEFIN FROM THE WITHDRAWN MIXTURE, PROVIDING AN EXCESS OF THE FREE MONOCARBOXYLIC ACID IN THE RESULTING OL EFIN-FREE MIXTURE, ADDING TO SAID OLEFIN-FREE MIXTURE A QUANTITY OF WATER ABOUT SUFFICIENT TO FORM A WATER AZEOTROPE WITH ALL THE ESTER FORMED, HEATING THE RESULTING AQUEOUS MIXTURE IN A DISTILLATION ZONE, WITHDRAWING AN ESTER-WATER AZEOTROPE AS A DISTILLATE FRACTION FROM SAID DISTILLATION ZONE, ALSO WITHDRAWING FROM SAID DISTILLATION ZONE A BOTTOMS FRACTION CONTAINING THE FREE MONOCARBOXYLIC ACID AND RELATIVELY ANHYDROUS SULFURIC ACID, RECYCLING SAID BOTTOMS FRACTION TO THE ESTERIFICATION STEP, AND SEPARATING THE DISTILLATE FRACTION INTO AN AQUEOUS PHASE AND ORGANIC ESTER PHASE. 